"The one capacitor in series with the RF input at the shack end probably wasn’t doing much blocking with transients that fast."
That capacitor will block dc, but the spike caused by the field collapsing around the relay coil is NOT dc. It is a cycle of ac. The spike will be coupled through the capacitor almost as if the capacitor wasn't there. Depending upon the coil inductance, current, etc., the spike it produces can easily reach into the hundreds of volts. It's a brief spike, but plenty to do a lot of damage. 73, Ron AC7AC -----Original Message----- From: Elecraft [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Dauer, Edward Sent: Saturday, May 21, 2016 9:32 AM To: elecraft@mailman.qth.net Subject: [Elecraft] K2 PA Failures and Remote Antenna Switch System For any who may be interested, the following is the resolution of a problem I had with a K2 that kept blowing its PA transistors – events for which I requested help on this reflector, and about which many replied (thank you.) Following the build last Fall the K2 blew away its PA transistors the first time it was connected to my antenna system. And, after repair, the second time and the third time. It went back to Don W three times for diagnosis and replacement of the transistors and other PA circuit components that failed when the transistors did. Don could not find any reason why the failures were occurring. With each replacement he checked it out thoroughly, on one trip for a full week, with no problems. When it came back to me each time I did the same on my bench with no problems. Then, each time, when I brought it to the operating QTH and put it on my antenna system, it failed in the same way. The supply voltage was doubly surge protected; the power supplies (different ones each time) included high-quality PowerWerx; the antennas are all resonant with SWRs below 1.3:1. The one common element, I finally realized, that was present with each failure but not with Don’s operating checks nor my bench checks was a remote antenna switch at my operating QTH which inserts its relay voltages into the RF feed line at the shack and picks it off at the remote relay box. On a hunch I looked at the voltages with a ‘scope and there it was – very high voltage transients with very short rise times, possibly attributable to the relays switching and the coil fields collapsing, though I confess the physics of it is above my pay grade. Inside the relay box I found no diodes and no capacitors across the relays that might have been used in the design to avoid this kind of problem. The one capacitor in series with the RF input at the shack end probably wasn’t doing much blocking with transients that fast. Although I have had no problem with a K3/KPA500 or a KX3/KXPA100 operating on the same system, the remote switch was the last remaining candidate for the repeated K2 PA failures. I have now discarded the Ameritron (MFJ) RCS-4 switch and replaced it with an Array Solutions “RatPak 6”, which uses an independent multi-conductor cable for its relay voltages. This is now the second weekend operating with the RatPak. No problems with the K2, at last. I can’t prove it, but there is reason to believe that the K2 had been damaged on multiple occasions by a remote antenna switch that inserted its relay coil voltages into the RF coax and which had circuit features inadequate to protect against the voltage transients getting back into the RF feed. Never again. Ted, KN1CBR ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
